Pressure sensitive adhesives for use on low energy surfaces

ABSTRACT

A vinyl graphic marking film or label coated with a pressure sensitive acrylic adhesive designed to achieve high or permanent adhesion to polymeric low energy surfaces is disclosed. In addition to good adhesion to low energy surfaces, the graphic marking film or label may have microperforated or micro-rough edges which initiate tearing when removal is attempted. The article maintains flexibility and film strength for easy fabrication in graphic marking film or label manufacture.

This application is a continuation of application Ser. No. 08/606,988filed Feb. 26, 1996, now abandoned.

FIELD OF THE INVENTION

This invention is in the field of adhesives, and more particularly,acrylic pressure sensitive adhesives which have improved bondingcharacteristics to low energy surfaces.

BACKGROUND

As used herein, the term "film" refers to a thin, flexible, single- ormulti-layer polymeric sheet. The term is used interchangeably with theterms "backing" and "carrier web". Graphic marking films or labelsformed from vinyl films coated with acrylic pressure sensitive adhesives(PSAs) are well-known in the art. However, to date, the ability toprovide such label and graphic marking film films with high bondingstrength, (i.e. non-removal), to low energy surfaces such as highdensity polyethylene and polypropylene plastics has not been possiblewithout sacrificing important properties of the adhesive such as shearstrength and cohesive strength.

As used herein, the term "low energy surfaces" is intended to mean thosesurfaces which exhibit low polarity and low critical surface tension(less than about 40 dynes/cm²) characteristics. One example of a lowenergy surface is the surface of a polyolefin plastic. Among the PSAs,it is known that acrylic-based PSAs exhibit poorer bond characteristicsto low surface energy polyolefin plastics than do rubber-based PSAs.This effect results from the greater difference in polarity between theacrylic PSA and the polyolefin surface as compared to that between therubber PSA and the polyolefin surface. Unfortunately, however, it wouldbe very desirable to use acrylic PSAs in many applications, sinceacrylic PSAs exhibit excellent outdoor durability, whereas rubber PSAsshow poor ultraviolet and oxidative stability due to chemicalunsaturation of the hydrocarbon elastomer.

Common physical methods to obtain high bond strength of pressuresensitive adhesives to polyolefin plastics include flame treating, whichoxidizes the surface of the plastic, chemical etching with strong acidsto increase polarity of the bonding surface, or the use of a primer ortopcoat containing a chlorinated polyolefin. For example, JapanesePatent No. HEI 1(1989)-242676 discusses the use of chlorinatedpolyolefin resins in connection with pressure sensitive adhesives. Onedisadvantage of such surface treatment methods is that they areinefficient in that they add an additional process step when applyinggraphic marking films or labels to low energy surfaces. This is lessthan ideal since industrial consumers of graphic marking films andlabels desire that these products can be easily applied in a single stepwithout the need for additional, time consuming surface preparationmethods.

For a permanent graphic marking film or label application, it would bedesirable to have bond characteristics similar to those associated withrubber-based PSAs combined with the outdoor durability associated withacrylic PSAs. One method to increase bond strength of acrylic PSAs tolow surface energy polyolefin plastics is to incorporate a compatibletackifier such as a rosin ester, a terpene phenolic resin or ahydrocarbon resin into the adhesive. Although the use of a tackifierdramatically improves bond strength as measured by peel force at lowspeeds, inclusion of these tackifiers raises the glass transitiontemperature (Tg) of the PSA which results in reduced low temperatureperformance and also causes a "shocky" or "zippy" peel characteristic atfaster peel rates. This "shocky" or "zippy" peel is an undesirablecharacteristic which can result in easy removal of films and labels, aswell as making them less tamper-resistant.

Additionally, loss in adhesive shear strength and cohesive strength isalso observed if large amounts of tackifier or plasticizer areincorporated into the PSA.

In many applications, it is desirable that graphic marking films orlabels be difficult to remove once they are applied to a surface. Thismay be accomplished by providing the graphic marking film or label filmwith a means by which attempts to remove it will result in tearing orother damage to the graphic marking film or label. A method to increasethe destructibility of PSA coated vinyl labels or graphic marking filmsis to make the vinyl film backing less elastic or "brittle". This isachieved by adding a hard acrylic resin and decreasing the plasticizerlevel of the vinyl film.

U.S. Pat. Nos. 5,141,790 (Calhoun et al.) and 5,296,277 (Wilson et al.)the teachings of both of which are incorporated herein by reference,describe an adhesive film referred to commercially as Controltac Plus®film (available from Minnesota Mining and Manufacturing Company,hereafter "3M"). The adhesive surface of the Controltac Plus™ film ischaracterized in that it includes clustered domains of a non-adhesivematerial, referred to as "pegs" which extend a short distance from theadhesive surface. The patents also describe adhesive films in which theadhesive surface is microtextured or provided with a microtopologicalstructure.

SUMMARY

The present invention relates to the incorporation of a plasticizermaterial into a tackified acrylic PSA. The resulting PSA is shown toprovide improved bonding of polyvinylchloride graphic marking films andlabels to low energy surfaces such as high density polyethylene plastic.

More specifically, the invention relates to tackified and plasticizedacrylic PSA compositions comprising:

a) about 100 parts by weight of an acrylic copolymer, said acryliccopolymer comprising from about 70-98% by weight of one or moremonofunctional acrylates having nontertiary alkyl groups with between 1and 14 carbon atoms and from about 30-2% by weight of a polar monomer;

b) about 10-40 parts by weight of a tackifier;

c) about 3-10 parts by weight of a plasticizer; and

d) optionally, a crosslinker.

The invention also relates to films which incorporate such tackifiedadhesives, and the use of those films as graphic marking films andlabels.

Specific chemical classes of plasticizers, when incorporated into thetackified pressure sensitive acrylic adhesive, improve the bondingcharacteristics of the adhesive to low energy surfaces. The addition ofplasticizer to a tackified acrylic adhesive has been found to improvethe wet-out or "quick stick" to low energy surfaces. The addition ofplasticizer offsets the increase in glass transition temperature causedby the tackifier and improves bond strength. When incorporated at levelsof less than about 10 parts plasticizer per 100 parts adhesive, thepresence of the plasticizer results in little effect on shearperformance or cohesive strength of the adhesive.

Among the plasticizers discovered to improve bonding characteristics arepolyglycol ethers, polyethylene oxides, phosphate esters, aliphaticcarboxylic acid esters, benzoic esters, and combinations thereof. Inaddition, other plasticizers which improve bonding to low energysurfaces have been identified, however, these have been found to causesome decrease in cohesive strength. These include sulfonamides andaromatic carboxylic acid esters.

The incorporation of the specified plasticizers along withacrylic-compatible tackifiers allows one to construct an outdoor durablevinyl label or graphic marking film with improved adhesion to low energysurfaces, and in particular, high density polyethylene surfaces, withoutthe need for physical or chemical treatment of the surfaces. In oneembodiment, the label or graphic marking film construction may include adestructible vinyl film backing for vandal-resistant applications. Inthis embodiment, the graphic marking film or label can be die cut with amicroperforated or micro-rough steel rule which will initiate tearingwhen removal is attempted. Removal is farther deterred by the improvedadhesion characteristics of the graphic marking film or label. Despitethese modifications to the film, the article maintains flexibility andfilm strength for easy fabrication in graphic marking film or labelmanufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the cross-section of a vinylfilm label.

FIG. 2 is a histogram plotting 180° peel adhesion for various adhesivecompositions.

FIG. 3 is a histogram plotting film shrinkage for various adhesivecompositions.

DETAILED DESCRIPTION OF THE INVENTION

Acrylic Pressure Sensitive Adhesives

A variety of acrylic pressure sensitive adhesives commonly used incommercial applications for vinyl decorating graphics films includeacrylic copolymers having from about 70-98% by weight of one or moremonofunctional acrylates having nontertiary alkyl groups with between 1and 14 carbon atoms and from about 30-2% by weight of a polar monomer.

In a preferred embodiment, the acrylic PSA is a copolymer ofethylenically-unsaturated alkyl acrylates (C₁ -C₁₄) such asisooctylacrylate, 2-ethylhexyl acrylate, 2-methylbutylacrylate (MBA),N-butyl acrylate, methylacrylate (MA), ethylacrylate, andisobornylacrylate (IBA). The polar monomer can compriseethylenically-unsaturated carboxylic acids such as methacrylic acids,acrylic acids (AA), itaconic acids, β-carboxyethylacrylates, fumaricacid, acrylamides (Acm) or other polar monomers such as n-vinylpyrrolidone, N-vinyl caprolactam, 2-hydroxyethyl acrylate, and the like.

This class of adhesives bonds very aggressively to painted substratesand has excellent cohesive strength due, at least in part, to the polarmonomer. While the use of polar monomers leads to adhesives with goodcohesive strength, the high glass transition temperature contribution ofthese monomers to the adhesive is a disadvantage when formulatingadhesives for "quick wet-out" on low energy surfaces.

One preferred PSA formulation for low energy surfaces was determined tobe a 95/5 ratio isooctylacrylate/acrylic acid. This formulation resultedin a composition which was balanced between too much acrylic acid (whichtended to "stiffen" the PSA formulation, leading to poor "wet-out"), andtoo little acrylic acid (which leads to PSA compositions with poorcohesive strength).

Several pressure sensitive adhesives having applicability to the presentinvention are presented in Table I below:

                  TABLE I    ______________________________________    Acrylic Pressure Sensitive Adhesives    Polymer Chemistry   Molecular Weight    ______________________________________    90/10        IOA/AA     1.5 × 10.sup.6    93/7         IOA/AA     1 × 10.sup.6    93/7         IOA/AA     5 × 10.sup.5    94/6         IOA/AA     5 × 10.sup.5    95/5         IOA/AA     2 × 10.sup.5    98/2         IOA/AA     1 × 10.sup.6    70/22.5/7.5  IOA/MA/AA  2 × 10.sup.5    96/4         2-MBA/Acm  2 × 10.sup.5    90/10        2-MBA/AA   5 × 10.sup.5    68/28/4      IOA/IBA/AA 2 × 10.sup.5    ______________________________________

Tackifiers

To obtain high bonding characteristics to low energy surfaces, the mostcommonly used tackifiers in acrylic pressure sensitive adhesives includeterpene phenolics, rosins, rosin esters, esters of hydrogenated rosins,synthetic hydrocarbon resins and combinations thereof. The tackifierswhich were evaluated are listed in the following Table II:

                  TABLE II    ______________________________________    Common Acrylic PSA Tackifiers    Company  Trade Name Chemical Class    ______________________________________    Hercules Foral ® 85                        Hydrogenated Glycerol Ester of Rosin    Dercules Hercolyn ®             Hydrogenated Methyl Ester of Rosin    Arizona  Nirez 2019 Terpene Phenolic Resin    Chemical    Union Camp             Uni-Tac-70 Tall Oil Rosin    Hercules Regalrez 6108                        Aromatic Hydrocarbon Resin    Exxon    ECR-180    Petroleum Hydrocarbon Resin    ______________________________________

Hydrogenated rosin esters are the preferred tackifiers as a result ofperformance advantages which include: high levels of "tack", outdoordurability, oxidation resistance, compatibility with hot meltprocessing, and limited interference in post crosslinking of acrylicPSAs.

Tackifiers are typically added at a level of about 10-40 parts per 100parts of dry acrylic PSA to achieve desired "tack". However, as notedabove, the addition of these tackifier types can reduce shear orcohesive strength and raise the Tg of the acrylic PSA which isundesirable.

Crosslinkers

In order to increase shear or cohesive strength of acrylic pressuresensitive adhesives, a crosslinking additive is usually incorporatedinto the PSA. Two main types of crosslinking additives are commonlyused. The first crosslinking additive is a thermal crosslinking additivesuch as a multifunctional aziridine. One example is 1,1'-(1,3-phenylenedicarbonyl)-bis-(2-methylaziridine) (CAS No. 7652-64-4), referred toherein as "Bisamide". Such chemical crosslinkers can be added intosolvent-based PSAs after polymerization and activated by heat duringoven drying of the coated adhesive.

In another embodiment, chemical crosslinkers which rely upon freeradicals to carry out the crosslinking reaction may be employed.Reagents such as, for example, peroxides serve as a precursor source offree radicals. When heated sufficiently, these precursors will generatefree radicals which bring about a crosslinking reaction of the polymerchains. A common free radical generating reagent is benzoyl peroxide.Free radical generators are required only in small quantities, butgenerally require higher temperatures to complete the crosslinkingreaction than those required for the bisamide reagent.

The second type of chemical crosslinker is a photosensitive crosslinkerwhich is activated by high intensity ultraviolet (UV) light. Two commonphotosensitive crosslinkers used for hot melt acrylic PSAs arebenzophenone and 4-acryloxybenzophenone which is copolymerized into thePSA polymer. Another photocrosslinker, which can be post-added to thesolution polymer and activated by UV light is a triazine; for example2,4-bis(trichloromethyl)-6-(4-methoxy-phenyl)-s-triazine. Thesecrosslinkers are activated by UV light generated from artificial sourcessuch as medium pressure mercury lamps. Depending on the type ofcrosslinker, no more than about 0.5% by weight of chemical crosslinkertypically is needed to achieve the desired crosslinking.

Aside from thermal or photosensitive crosslinkers, crosslinking may alsobe achieved using high energy electromagnetic radiation such as gamma ore-beam radiation.

A physical crosslinking agent may also be used. In one embodiment, thephysical crosslinking agent is a high T_(g) macromer such as those basedupon polystyrene and polymethylmethacrylate which are used at about 2-6parts by weight per 100 parts by weight dry adhesive.

Diisocyanates have also been reported as crosslinking agents foradhesives based on copolymers of acrylic acids.

Other Additives

Since acrylic pressure sensitive adhesives have excellent oxidativestability, additives such as antioxidant and UV light absorbers aregenerally not needed. In contrast, rubber-based PSAs typically includesuch additives.

Small amounts of heat stabilizer, (less than about 0.3% by weight), canbe utilized in hot melt acrylic PSAs to increase thermal stabilityduring processing.

Although not required, in some special applications, fillers (clay) orcolorants (TiO₂ or carbon black) may be used as additives to impartopacity or color to the adhesive, or to make the adhesive system lessexpensive.

Backings

In one embodiment of the present invention, backings of plasticized,flexible polyvinylchloride films are used to form decorative films,labels or graphic marking films. A modified polyvinylchloride film is ofinterest for destructible or vandal resistant type labels or graphicmarking films. This film has been made "brittle" or "tearable" bylowering the plasticizer level and adding a methylacrylate/butylacrylatecopolymer resin (Elvacite™ 2013 available from Imperial ChemicalIndustries, Wilmington, Del.) to the PVC film formulation. These filmsmay be made by casting from organosol solutions or calendered from anextrudable PVC resin.

Other backings of interest include, but are not limited to, polyesters,polyolefins, papers, foils, polyacrylates, polyurethanes,perfluoropolymers, polycarbonates, ethylene vinyl acetates, and thelike. Backings of vinyl films, woven and nonwoven sheets, woven andnonwoven fabrics, papers and retroreflective sheeting are intended to beincluded.

Plasticizers

Plasticizers useful in the invention are selected from a wide variety ofcommercially available materials. Representative plasticizers are listedin Table III.

                  TABLE III    ______________________________________    Acrylic PSA Plasticizers    Company           Trade Name  Chemical Class                                     Chemical Name    ______________________________________    ICI    Pycal 94    Polyethylene Oxide                                     Polyoxy-    Americas                         ethylene                                     Aryl Ether    Monsanto           Santicizer 97                       Adipic Acid Ester                                     Dialkyl                                     Adipate    Monsanto           Santicizer 141                       Phosphoric Acid                                     2-Ethylhexyl                       Ester         Diphenyl                                     Phosphate    Monsanto           Santicizer 54                       Phosphoric Acid                                     t-Butylphenyl                       Ester         Diphenyl                                     Phosphate    Monsanto           DOA         Adipic Acid Ester                                     Di(2-                                     Ethylhexyl)                                     Adipate    Akzo   Ketjenflex 8                       Sulfonamide   Toluene-    Nobel                            sulfonamide    Velsicol           Benzoflex 9-88                       Benzoic Acid Ester                                     Dipropylene                                     Glycol                                     Dibenzoate    Velsicol           Benzoflex P-200                       Benzoic Acid Ester                                     Poly-                                     ethylene                                     Glycol                                     Dibenzoate    Rhone- Alkapol SQR-490                       Polypropylene Oxide                                     Poly-    Poulenc                          oxypropylene                                     Aryl Ether    Sartomer           Sartomer 660                       Formic Acid Ester                                     Dibutoxy-           (Cryoflex)                ethoxyethyl                                     Formal    Sartomer           Sartomer 650                       Adipic Acid Ester                                     Dibutoxy-           (Wareflex)                ethoxyethyl                                     Adipate    ______________________________________

In each case, the added plasticizer must be compatible with the acrylicPSA being used in the formulation. The amount of plasticizer added tothe PSA formulation is dependent upon the molecular weight of theadhesive. It is preferred that a minimum amount of plasticizer be used.Specifically, it is one feature of the invention to obtain improvedsubstrate bonding characteristics without sacrificing cohesive strengthor shear strength performance. Typically, as much as about 10 parts byweight plasticizer per 100 parts adhesive can be added withoutcompromising cohesive strength for high molecular weight, (Mw greaterthan about 1×10⁶), adhesives. In the case of lower molecular weight, (Mwless than about 3×10⁵), no more than about 5 parts by weight per 100parts adhesive is needed.

Method of Making Plasticized Acrylic PSAs

Plasticizer can be added to the acrylic pressure sensitive adhesives atnumerous points during the adhesive formulation process. For example,the plasticizer may be added to the acrylic copolymer or terpolymersolutions either prior to or following polymerization, it may be blendedinto the melt for hot-melt acrylic PSAs during coating, it may be addedto the monomer syrup of ultraviolet- or thermally- polymerized acrylatePSAs, or it may be added to a water-based acrylate PSA emulsion.

For these methods, it is preferred that the plasticizer is miscible withthe acrylate PSA or its solution, is soluble in any solvents that arepresent, is thermally stable in hot melt applications, does notsubstantially interfere with the polymerization reaction or thecrosslinking process, and forms a stable emulsion along with theacrylate PSA in water-based adhesive formulations.

The present invention has particular applicability in connection withvinyl films for graphic marking film and labels. In one embodiment, thevinyl film can be an extendible polyvinylchloride backing for use indecorative labels and graphic marking films. As noted above, atamper-resistant vinyl film may also be made. Such tamper-resistantfilms have particular applicability for use as warning, instruction orsafety labels which break apart when tampered with, making the labeldifficult or impossible to remove.

A typical film construction is shown in FIG. 1 in which the film 10comprises a film layer 12, such as a 2 mil (0.05 mm) vinyl/acrylate filmhaving an acrylic PSA 14 applied to one surface thereof. A release liner16 such as silicon-coated paper or film is adhered to the PSA until thefilm is ready to be adhered to a surface.

The film may be made tamper-resistant by die cutting the film layer 12with a 60T microperforated steel rule. The "micro-rough" edges impartedto the film enhance its "tear" or "breaking" characteristics.

Films of the type depicted in FIG. 1 can be applied to low energyplastic surfaces such as high density polypropylene (HDPP) and highdensity polyethylene (HDPE) parts. Such parts are useful in many outdoorapplications including, but not limited to, lawn and garden equipment,recreational vehicles, all-terrain vehicles, snowmobiles, motorcyclesand watercraft. The particular parts include, but are not limited to,covers, hoods and fenders which are decorated with films forming vinylgraphic marking films, warning labels and instructional labels.

For the films of the present invention, measured peel adhesions at amoderate peel rate of about 90 inches (230 cm) per minute should begreater than 4 pounds per inch (18N/25 mm width) to achieve adequatebonding characteristics to high density polyethylene. Dead load shearperformance of 10,000 minutes or greater is desired as measured by thePSTC-7 Static Shear Test (1.6 cm² ×1.6 cm² /1 kilogram) described below.However, for applications in which vinyl graphic marking film or labelfilms are applied to the parts described above, this level of dead loadshear performance is not required. It is preferable, however, that thePSA has sufficient adhesive strength to resist shrinkage forces whichmay be imparted by the graphic marking film or label film to which theadhesive is applied.

EXAMPLES

Testing Protocol

1. 180° Peel Adhesions PSTC-1: Based on the Pressure Sensitive TapeCounsel (PSTC) test standard, a 1 inch (2.54 cm) wide strip of PSAcoated 2 mil (0.05 mm) vinyl film is laminated to a high densitypolyethylene test panel. The test specimen is backed with a standard 2mil vinyl film for reinforcement. The applied test specimen is allowedto equilibrate for about 24 hours at about 72° F. (22° C.) and about 50%relative humidity. The test sample is than peeled at an angle of 180°and a speed of about 12 or 90 inches (30 or 230 cm) per minute, using aLloyd, Instron or IMASS peel test machine. The peel adhesion measurementis reported in pounds per inch width or kilograms per 2.54 centimeterswidth.

2. Static Shear PSTC-7: A 1/2 inch square (1.6 cm²) sample PSA coatedrigid foil or polyester film is laminated to #304 Stainless Steel totest for holding power or cohesive strength. The sample is allowed toequilibrate for about 24 hours at about 72° F. (22° C.) and about 50%relative humidity before a 1 kilogram weight is applied. The test is runat about 72° F. (22° C.) and about 50% relative humidity conditions. Thetime to fail, when the sample separates from the panel in minutes isrecorded. The failure mechanism is also recorded which is either"pop-off" wherein no adhesive residue remains on the panel or thebacking, or "cohesive" wherein adhesive remains on both test panel andtest sample.

3. Film Shrinkage: This test is an additional test used to measureinternal adhesive or shear strength. It relates directly to actualproduct use conditions. This test measures the ability of the PSA to"hold the vinyl film in place" or to resist the shrinkage forcesimparted by the vinyl film. A 21/2 inch (6.35 cm) by 4 inch (10.2 cm)PSA coated vinyl film sample is applied to an aluminum panel. Theapplied vinyl film sample is slit with a razor blade in both thecrossweb and machine direction and is conditioned at 150° F. (65.6° C.)for 24 hours. Measurements in one-one thousand inch (mils) increments ofthe razor cut openings are recorded. The razor slit will tend toseparate or widen for adhesives with poor internal or cohesive strength.Generally a gap opening of 10 mils or greater indicates that the PSA haspoor shear strength while adhesives that have good shear or cohesivestrength will show small gap openings, less than 10 mils, and will holdthe vinyl film in place.

Example 1--Solution Cast Sample

A pressure sensitive acrylic adhesive for low energy surfaces wasformulated by adding to 100 parts (solids) of a high molecular weight,(Mw greater than 1×10⁶), 93/7 isooctylacrylate/acrylic acid (IOA/AA)copolymer solution (25% solids in ethyl acetate), 20 parts Foral®, 10parts Hercolyn®-D, and 100 parts Pycal® 94. To this formulation, 0.6parts Bisamide crosslinker per 100 parts adhesive (5% solution intoluene) is added. The solution was mixed for 1 minute and rolled for anadditional hour to reduce air entrapment. The adhesive composition iscoated onto silicone release liner, and then dried in an oven for 5minutes at a temperature of 200° F. (93.3° C.). A coating weight of 0.6g/24 in² (155 cm²) was targeted.

The coated pressure sensitive adhesive of this example was laminated tovarious films to produce labels and graphic marking films. In oneembodiment, the film may be a polyvinylchloride film which is madeflexible by the presence of a PVC-compatible plasticizer or,alternatively, the film may be a brittle, destructible polyvinylchloridefilm which contains both a PVC-compatible plasticizer and a copolymerresin, such as a methylacrylate/butylacrylate copolymer resin.

Example 2--Hot Melt Sample

A pressure sensitive acrylic adhesive coated vinyl graphic marking filmfor low energy surfaces was made by formulating an adhesive with acomposition of 100 parts by weight of a low molecular weight, (Mw lessthan 3×10⁵), 95/5/0.4 isooctylacrylate/acrylicacid/4-acryloxybenzophenone terpolymer, 15 parts by weight Foral®85, 5parts by weight Hercolyn®-D, and 5 parts by weight Pycal® 94.

This formulation was hot melt coated onto a silicone release liner at acoating weight of 0.6 g/24 in² (155 cm²) and exposed to a UV light doseof 525 mj/cm². (National Institute of Standards and Technology inaccordance with EIT and MIL-STD 45662A). A pressure sensitive adhesivecoated vinyl film was prepared as described in Example 1.

Performance Data

The data in FIG. 2 shows the effects that tackifier and plasticizer haveon peel adhesion on high density polyethylene plastic. 180° peeladhesion values greater than 17 Newtons per 25 mm width, at a peel rateof 30 cm/minute, are realized when a tackified pressure sensitiveadhesive is formulated with Pycal®94 plasticizer. With an increased peelrate of 230 cm/minute, the Pycal®94 plasticized PSA exhibits peeladhesions greater than 20 newtons/25 mm width. These results are uniquein that the highest peel adhesions are observed for tackified acrylicPSAs which have been plasticized with Pycal®94. At a fast peel rate of230 cm/minute, the difference in 180° peel adhesions on high densitypolyethylene are even greater for Pycal®94 modified acrylic adhesivesthan those adhesives without plasticizer.

Internal adhesive strength is another important performancecharacteristic. In applications where vinyl graphic films or decals arecoated with acrylic PSA, the adhesive must have enough internal strengthto resist the inherent tendency of the vinyl to shrink. Adhesivestrength is most commonly measured by a dead load or static shear test,as described in PSTC-7. An alternative method for determining adhesivestrength is to measure the film shrinkage of the PSA-coated vinyl film.

The data in FIG. 3 shows how film shrinkage is effected by the additionof the Pycal®94 plasticizer tackified acrylic PSA. The addition of thePycal®94 to the PSA formulation does cause a minor change in filmshrinkage. However, this slight change is negligible in the overallperformance of the vinyl coated PSA.

The data in Table IV shows that different classes of plasticizers may beutilized to achieve the desired performance characteristics. The dataincludes 180° peel adhesions to high density polyethylene, filmshrinkage measurements with and without solvent containingscreenprinting inks, and static shear values. The data shows thatdifferent classes of plasticizers, when incorporated into the PSAformulation, exhibit peel adhesions over 15 Newtons/25 mm width on highdensity polyethylene while maintaining film shrinkage performance ofless than 0.25 mm.

Shrinkage measurements for PSA coated vinyl films printed with solventinks were also recorded in Table IV. The ability of the PSA coated vinylfilms to resist the effects of solvent ink printing on film shrinkage isimportant to product performance. Typically, film shrinkage values ofPSA coated vinyl films, printed with solvent inks, are double incomparison to non-screenprinted PSA coated vinyl films. In Table IV,under Film Shrinkage, 3900 Solvent Ink, the film was coated with ascreen printing ink and a transparent coating. In the first step of theprocess, a black solvent ink (Scotchcal™ 3905 available from 3M) isscreen printed onto a film using a 225-mesh polyester screen. Theprinted film is dried in a forced draft oven for 1 hr at 150° F. (66°C.). In the second step of the process, the black-printed film is thenovercoated with a clear coat (Scotchcal™ 3920 available from 3M) usingthe same screen and dried an additional 1 hr at 150° F. (66° C.).

The static shear data in Table IV correlate with the film shrinkage testresults. Those plasticized PSA samples with less than 10,000 minutes ofshear exhibit cohesive or internal adhesive failure. These measurementswere made at 23° C,. using a 1000 gram weight with a sample area of 1.6cm². Also shown in Table IV are plasticizers with high film shrinkage orlow static shear values. Although not wishing to be bound by anyparticular theory, this may indicate incompatibility of thoseplasticizers with the acrylic PSA.

In summary, the data presented in Table IV and FIGS. 2 and 3 shows thatincreased peel adhesions to low energy surfaces, such as high densitypolyethylene, can be achieved with addition of tackifiers and specificplasticizers without compromising internal adhesive strength.

                                      TABLE IV    __________________________________________________________________________    95/5/0.4 IOA/AA/ABP Polymer with 15 Parts Foral ® 85 & 5 Parts    D TackifiersM.              180° Peel Adhesion (N/25 mm)                            Film Shrinkage (mm)                                        STATIC SHEAR @23° C. (minute)    Plasticizer              30 cm/minute                     230 cm/minute                            No Ink                                3900 Solvent Ink                                        Chromate Primed Aluminum    __________________________________________________________________________    5.0% Pycal 94              19.8   22.9   0.150                                0.250   10,000+    5.0% Santicizer 97              11.0   19.4   0.175       10,000+    5.0% Santicizer 141              15.4   21.6   0.175                                0.250   10,000+    5.0% Santicizer 154              16.3   21.6   0.175                                0.275   10,000+ Pop-off    5.0% DOA  12.3   0.0    0.175                                0.275   10,000+    5.0% Ketjenflex 8              16.7   18.0   0.250                                0.650   80      Cohesive    5.0% Benzoflex P-200              15.4   21.6   0.175                                0.325   1,500   Cohesive    5.0% Benzoflex 9-88              15.4   21.1   0.175                                0.375   90      Cohesive    5.0% Alkopol SQR-490              17.2   20.2   0.200                                0.300   10,000  Pop-off    5.0% Sartomer 660              14.1   22.0   0.200                                0.325   10,000+    5.0% Sartomer 650              12.3   20.7   0.150                                0.300   10,000+    __________________________________________________________________________

The data in Table V, below, represents a diverse range of PSA copolymersand terpolymers that were modified with plasticizer to enhance adhesionto low energy plastics. Pressure sensitive adhesives made fromcopolymers of isooctylacrylate/acrylic acid (IOA/AA),2-methylbutylacrylate/acrylic acid (2-MBA/AA) and2-methylbutylacrylate/acrylamide (2-MBA/Acm) and terpolymers ofisooctylacrylate/methylacrylate/acrylic acid (IOA/MA/AA) andisooctylacrylate/isobornylacrylate/acrylic acid (IOA/IBA/AA) weretested. Pressure sensitive adhesive formulations 1-14 were made by bulksolution polymerization. Formulation 15 was made by solventlessultraviolet polymerization. Average molecular weight (M_(w)) wasdetermined via GPC. Various tackifiers and crosslinking methods wereutilized in the PSA formulations.

180° peel adhesions to high density polyethylene increase when Pycal®94plasticizer is added to the PSA. For example, when 5 parts of Pycal®94is added to 100 parts of a copolymer consisting of 96 parts2-methylbutylacrylate and 4 parts acrylamide, the adhesion to the highdensity polyethylene substrate increases from 2.2N/25 mm width to6.6N/25 mm width at a peel rate of 230 cm/minute. Peel adhesionsincreased from 4.4N/25 mm width to 17.2N/25 mm width at a peel rate of30 cm/minute when 7.5 parts of Pycal®94 is added to 100 parts ofadhesive terpolymer consisting of 76 parts of isooctylacrylate, 21.5parts of isobornylacrylate and 2.5 parts of acrylic acid.

                                      TABLE V    __________________________________________________________________________                                                180° Peel Adhesion                                                (N/25 mm)                                                         Film Shrinkage    PSA Formulation     Tackifier Plasticizer   30 cm/                                                    230 cm/                                                         (mm)    100 Parts    Crosslinker                        Parts     Parts    Mw   minute                                                    minute                                                         No    __________________________________________________________________________                                                         Ink    1 98/2 IOA/AA                 0.6% Aziridine                        25 Foral ® 85                                  5 Pycal ® 94                                           1 × 10.sup.6                                                17.2                                                    20.7 0.350    2 95/5 IOA/AA                 0.3% ABP                        11 Regalrez 6108                                  5 Pycal ® 94                                           2 × 10.sup.5                                                    13.2 0.125    3 95/5 IOA/AA                 0.4% ABP                        15 Foral ® 85/5                                  5 Pycal ® 94                                           2 × 10.sup.5                                                19.8                                                    22.9 0.200    D                   Hercolyn ®    4 93/7 IOA/AA                 0.6% Aziridine                        25 ECR-180/10                                  10 Pycal ® 94                                           1 × 10.sup.6                                                15.0                                                    23.3 0.075    D                   Hercolyn ®    5 94/6 IOA/AA                 1.5% Azirdine                        12 Nirez ® 2019/7.5                                  11 Pycal ® 94                                           5 × 10.sup.5                                                    20.2 0.150                        Unitac ® 70    6 90/10 2-MBA/AA                 1.1% Aziridine                        None      None     5 × 10.sup.5                                                2.2 0.9  0.125    7 90/10 2-MBA/AA                 1.1% Azirdine                        None      5 Pycal ® 94                                           5 × 10.sup.5                                                11.0                                                    2.2  0.150    8 96/42-MBA/Acm                 0.2% ABP                        None      None     2 × 10.sup.5                                                7.9 2.2  0.175    9 96/42-MBA/Acm                 0.2% ABP                        None      5 Pycal ® 94                                           2 × 10.sup.5                                                10.6                                                    6.6  0.300    10      76/21.5/2.5 IOA/IBA/AA                 0.6% Aziridine                        20 Regalrez ® 6108                                  None     1 × 10.sup.6                                                4.4 2.2  0.100    11      76/21.5/2.5 IOA/IBA/AA                 0.6% Aziridine                        20 Foral ® 85                                  7.5 Pycal ® 94                                           1 × 10.sup.6                                                17.2                                                    17.6 0.150    12      76/21.5/2.5 IOA/IBA/AA                 0.6% Aziridine                        20 Regalrez ® 6108                                  7.5 Pycal ® 94                                           1 × 10.sup.6                                                17.2                                                    3.5  0.125    13      70/22.5/7.5 IOA/MA/AA                 0.15% ABP                        None      5 Pycal ® 94                                           2 × 10.sup.5                                                13.6                                                    11.0 0.200    14      85/10/5 IOA/MA/AA                 0.4% ABP                        15 Foral ®/5                                  5 Pycal ® 94                                           2 × 10.sup.5                                                19.4                                                    23.3 0.125    D                   Hercolyn ®    15      90/10 IOA/AA                 0.2% Triazine                        None      4 Santicizer ® 141                                           1.5 × 10.sup.6                                                6.6      0.250    __________________________________________________________________________

Equivalents

Various modifications and alterations to this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention. It should be understood that thisinvention is not intended to be unduly limited by the illustrativeembodiments and examples set forth herein and that such examples andembodiments are presented by way of example only with the scope of theinvention intended to be limited only by the claims set forth herein asfollows.

We claim:
 1. An acrylic pressure sensitive adhesive which comprises:a)about 100 parts by weight of an acrylic copolymer, said acryliccopolymer comprising from about 70-98% by weight of one or moremonofunctional acrylates having nontertiary alkyl groups with between 1and 14 carbon atoms and from about 30-2% by weight of a polar monomer;b) about 10-40 parts by weight of a tackifier; c) about 3-10 parts byweight of a plasticizer; and d) optionally, a crosslinker;wherein theeplasticizer is selected from the group consisting of polyoxyethylenearyl ethers, polyoxylpropylene aryl ethers, phosphate esters,sulfonamides, aromatic carboxylic acid esters, and combinations thereof.2. The acrylic pressure sensitive adhesive of claim 1 wherein themonofunctional acrylate is selected from the group comprisingethylenically-unsaturated alkyl acrylates.
 3. The acrylic pressuresensitive adhesive of claim 1 wherein the polar monomer is selected fromthe group consisting of ethylenically-unsaturated carboxylic acids andacrylamides.
 4. The acrylic pressure sensitive adhesive of claim 1wherein the tackifier is selected from the group comprising terpenephenolics, rosins, rosin esters, esters of hydrogenated rosins,synthetic hydrocarbon resins and combinations thereof.
 5. An adhesivefilm adapted for use on low energy surfaces which comprises:a) apolymeric backing; and b) a pressure sensitive adhesive applied to asurface of the polymeric backing, the pressure sensitive adhesivecomprising:i) about 100 parts by weight of an acrylic copolymer, saidacrylic copolymer comprising from about 70-98% by weight of one or moremonofunctional acrylates having nontertiary alkyl groups with between 1and 14 carbon atoms and from about 30-2% by weight of a polar monomer;ii) about 10-40 parts by weight of a tackifier; iii) about 3-10 parts byweight of a plasticizer; and iv) optionally, a crosslinker;wherein theplasticizer is selected from the group consisting of polyoxyethylenearyl ethers, polyoxypropylene aryl ethers, phosphate esters,sulfonamides, aromatic carboxylic acid esters, and combinations thereof.6. The adhesive film of claim 5 wherein the polymeric backing comprisesa material selected from the group consisting of vinyl films, woven andnonwoven sheets, woven and nonwoven fabrics, papers and retroreflectivesheeting.
 7. The adhesive film of claim 5 wherein the monofunctionalacrylate is selected from the group comprising ethylenically-unsaturatedalkyl acrylates.
 8. The adhesive film of claim 5 wherein the polarmonomer is selected from the group consisting ofethylenically-unsaturated carboxylic acids and acrylamides.
 9. Theadhesive film of claim 5 wherein the tackifier is selected from thegroup comprising terpene phenolics, rosins, rosin esters, esters ofhydrogenated rosins, synthetic hydrocarbon resins and combinationsthereof.
 10. A graphic marking or label having a visible surface andformed from the adhesive film of claim 5 wherein the visible surfaceconveys information or provides decoration.
 11. A graphic marking orlabel having a visible surface and formed from the adhesive film ofclaim 5 having edges which are microrough.
 12. The adhesive film ofclaim 5 which comprises a transfer adhesive having a backing comprisinga release liner.
 13. A double-sided tape comprising a polymeric filmhaving two surfaces, each surface having an adhesive thereon, wherein atleast one such adhesive comprises the adhesive of claim
 1. 14. A processfor making the adhesive of claim 1 which includes the steps of:a) hotmelt coating the adhesive; and b) curing the adhesive using ultravioletlight.
 15. A process for making the adhesive of claim 1 which includesthe steps of:a) casting a syrup of the adhesive; and b) curing theadhesive using ultraviolet light.
 16. The adhesive film of claim 5 whichfurther includes a release liner.
 17. The adhesive film of claim 16wherein the release liner has a microtextured surface.